Liquid-liquid extraction also known as solvent extraction and partitioning is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. Solvent extraction is commonly used in metal separation processes, allowing metals to be extracted from aqueous solutions by using organic extraction solutions. Industrially the largest application of solvent extraction is extraction of copper from aqueous sulfuric acid solutions following leaching of the copper ore.
In hydrometallurgy the aqueous solution from leaching, the pregnant leach solution, is contacted in a mixer-settler with an extractant, i.e. an organic extraction reagent dissolved in an organic diluent, to selectively transfer the desired metal from the aqueous phase to a water-immiscible phase. After phase separation, the aqueous solution, the raffinate, is recycled back to leaching and the organic water-immiscible phase is moved to another mixer-settler where it is contacted with an aqueous stripping solution to create a more concentrated, and more pure aqueous solution of the metal for reduction. Metal extraction requires usually two or more consecutive extraction stages to remove the target metals from the aqueous feed.
Concerning the technical performance of the solvent extraction process the mixers can be operated either aqueous continuous (i.e. organic droplets are dispersed into water) or organic continuous (vice versa). Due to various reasons one or the other gives more over-all optimal result for the solvent extraction process. The phase disengagement rate is highly dependent on the phase continuity and usually the organic continuous dispersion separates faster. However, in many cases the over-all process optimum is better with aqueous continuous system. Faster phase separation rates in a settler enable either smaller settlers, less organic phase inventory or higher flow rates and productivity in existing plants. For the operability and trouble shooting of a solvent extraction plant it is an advantage that the difference between aqueous and organic continuous systems is not too big especially in start-up situations or accidental phase inversion events.
The commonly used metal extraction reagents in solvent extraction of copper are various hydroxyoxime derivatives which are able to form a chelate with copper ion. This copper complex preferably distributes into the organic phase during the extraction stage.
Conventional diluents used in copper solvent extraction plants originate from fossil crude oil. A wide range of commercially available hydrocarbon diluents exists. One of the commonly used diluent is e.g. Shellsol® D70 which contains about 55% paraffinic and 45% naphthenic hydrocarbons with an average of 11-14 carbon atoms.
Today there is a global demand to replace fossil solvents and VOC (volatile organic compounds) containing diluents with alternatives due to environmental and regulatory pressures and health concerns as well as increasing price and unsteady support of crude oil.
In an attempt to find alternatives a possibility of using biodiesel such as “Fatty Acid Alkyl Esters” (FAAE) and especially “Fatty Acid Methyl Esters (FAME) as diluent in metal extraction has been considered. However, the physical and chemical properties of these are not suitable for metal extraction. Moreover, the esters are subject to hydrolysis in high acidic condition, which is the case e.g. in the solvent extraction of copper. Acid catalyzed hydrolysis of the esters results in a formation of the corresponding carboxylic acids which are known to act as metal extractant for some impurity metals.